Clean up a comment in processlog.py.

 * There is a problem with this justification for the ccache detection --
   the two g++ subcommands should already be filtered out.  The first lacks
   a -c argument, and the second command's tmp.i source file has a file
   extension not listed in sourceExtensions.  (If compiler drivers accept
   tmp.i as a valid source file, though, then maybe .i should be in
   sourceExtensions.)